The work is designed to elucidate the operations which comprise the perceptual constancy algorithm for perceived shape. The experiments seek for exploit the backward masking procedure to identify the stages underlying the attainment of constancy and to decribe their nature. A second line of research proceeding concurrently (or embedded) in the first will be to establish the masking functions for a number of different cues for rotation in depth. This latter line of work is related to the first, but will also be applied to an assessment of certain claims associated with the theory of direct perception stimulated by the writings of J.J. Gibson.